1. Field of the Invention
This invention relates to a process for recovering protein from an animal muscle source with improved functional properties and to the protein product so-obtained. More particularly, this invention relates to a process for recovering muscle proteins with improved functional properties from an animal source and the protein product so-obtained.
2. Description of Prior Art
Presently, there is an interest in expanding the use of muscle proteins as food because of their functional and nutritional properties. Better use of these materials would be particularly important with aged or frozen raw materials which are less valuable because they have lost protein functionality. It is presently believed that the muscle tissue utilized as the feed in present processes must be fresh rather than frozen or aged. It is common commercial practice to process freshly caught fish at sea on board ship rather than subject the fish to the time of transportation or the freezing necessary to effect processing on land. Ageing or freezing of fish lowers the functional qualities of the tissue proteins. Protein functionalities of most concern to food scientists are solubility, water holding capacity, gelation, fat binding ability, foam stabilization and emulsification properties.
Protein concentrates from muscle tissue, especially fish, have been made by hydrolysis. This approach has improved some functional properties, particularly solubility, which has allowed its use in prepared soups. However, this approach also destroys other functional properties such as gelling ability.
One process that has had some success in stabilizing protein foods has been the process for producing xe2x80x9csurimixe2x80x9d. This conventional process has been used primarily for fish, although there have been some attempts to produce a surimi-like product from other raw materials such as mechanically deboned poultry mince. In producing surimi, the fresh muscle is ground and washed with a variable amount of water a variable number of times. This is determined by the location of the plant and the product that is desired from the particular species. Water may be used in a ratio as low as about 2 parts water to one part fish up to about 5 parts water per 1 part fish; typically about 3 parts water are used per 1 part fish. The number of washes can vary, generally, from 2 to 5, again depending on the raw material, the product desired, and water availability. Twenty to thirty per cent of the fish muscle proteins are solubilized when the ground muscle is washed with water. These soluble proteins, known as sarcoplasmic proteins, are generally not recovered from the wash water of the process. This loss is undesirable since sarcoplasmic proteins are useful as food. The washed minced product containing the protein in solid form then is used to make protein gels. Originally, this was used to produce xe2x80x9ckamabokoxe2x80x9d in Japan. Kamaboko is a popular fish sausage in which the washed minced fish is heated until it gels. It is presently believed that it is necessary to add cryprotectants to the washed, minced fish before freezing to prevent protein denaturation. A typical cryoprotectant mixture comprises about 4% sucrose, about 4% sorbitol and about 0.2% sodium tripolyphosphate. These components retard the denaturation of the protein during freezing, frozen storage and thawing.
It has been proposed by Cuq et al, Journal of Food Science, pgs. 1369-1374 (1995) to provide edible packaging film based upon fish myofibrillar proteins. In the process for making the films, the protein of water-washed fish mince is solubilized in an aqueous acetic acid solution at pH 3.0 to a final concentration of 2% protein. No attempt was made in this work to re-adjust the pH values of the acidified proteins to re-establish the functional properties attained at pH values above about 5.5. In addition, the use of acetic acid imparts a strong odor to the material which would severely limit its use in a food product.
It also has been proposed by Shahidi and Onodenalore, Food Chemistry, 53 (1995) 51-54 to subject deboned, whole capelin to washing in water followed by washing in 0.5% sodium chloride, followed by washing in sodium bicarbonate. The series of washes, including that using sodium bicarbonate, would remove greater than 50% of the muscle proteins. Essentially all of the sarcoplasmic proteins would be removed. Final residue was further washed to remove residual bicarbonate. The washed meat was then suspended in cold water and heated at 70xc2x0 C. for 15 min. This heat treatment is sufficient to xe2x80x9ccookxe2x80x9d the fish proteins, thus denaturing them and reducing or eliminating their functional properties. No attempt was made to restore proteins to improve the functional properties of the capelin proteins.
Shahidi and Venugopal, Journal of Agricultural and Food Chemistry 42 (1994) 1440-1448 disclose a process for subjecting Atlantic herring to washing in water followed by washing with aqueous sodium bicarbonate. Again, this process will remove greater than 50% of the muscle proteins, including the sarcoplasmic proteins. The washed meat was homogenized and the pH varied between 3.5 and 4.0 with acetic acid. In addition, there is an unacceptable odor problem with the volatile acetic acid.
Venugopal and Shahidi, Journal of Food Science, 59, 2 (1994) 265-268, 276 also disclose a similar process for treating minced Atlantic mackerel. The material is washed sequentially with water, bicarbonate solution and again water. The pH is brought to pH 3.5 with acetic acid after homogenization. The proteins were precipitated at pH values greater than 4 on heating the material to 100xc2x0 C. for 15 min. It is disclosed that xe2x80x9cdissolution of structural proteins of fish muscle requires extractants with an ionic strength  greater than 0.3xe2x80x9d.
Shahidi and Venugopal, Meat Focus International, October 1993, pgs 443-445 disclose a process for forming homogenized herring, mackerel dispersions or capelin dispersions in aqueous liquids having a pH as low as about 3.0. It is reported that acetic acid reduces the viscosity of herring dispersions, increases viscosity of mackerel to form a gel and precipitates capelin. All of these preparations were initially washed with water and sodium bicarbonate, which would remove a substantial proportion of the protein, including the sarcoplasmic proteins.
Chawla et al, Journal of Food science, Vol. 61, No.2, pgs 362-366, 1996 discloses a process for treating minced threadfin bream muscle after it has been washed twice with water and recovered by filtration. The minced fish product is mixed with tartaric, lactic, acetic or citric acid, is allowed to set and then is heated in a boiling water bath for twenty minutes and then cooled to form a gel. This heat treatment is sufficient to denature the proteins. The washing steps undesirably remove soluble sarcoplasmic proteins from the mince. It is also disclosed that unwashed mince failed to provide the desired gel forming property of surimi.
Onodenalore et al, Journal of Aquatic Food Products Technology, Vol. 5(4), pages 43-59 discloses that minced shark muscle is a source of acidified protein compositions. The minced product is washed sequentially with aqueous sodium chloride, aqueous sodium bicarbonate and then water to remove metabolic substances. This washing effects undesirable removal of sarcoplasmic proteins. The minced product is recovered by filtration. The minced product then is acidified to pH 3.5 with acetic acid, heated in a boiling water bath, cooled and centrifuged to recover a supernatant. The supernatant pH was adjusted to a pH 4-10 using NaOH, heated in a boiling water bath, cooked and centrifuged to recover a second supernatant. Heating the protein dispersion comprising the minced product resulted in 87-94% of the protein remaining in solution while heating of the unacidified protein dispersion resulted in protein coagulation. However, the heating causes protein denaturation.
Accordingly, it would be desirable to provide a process for recovering a high proportion of available muscle protein from an animal source including a frozen or aged animal source, rather than requiring a fresh muscle tissue source. It would also be desirable to provide such a process, which permits the use of muscle protein sources which are presently under-utilized as a food source such as frozen or aged fish. Furthermore, it would be desirable to provide such a process which recovers substantially all of the protein content of the process feed material. In addition, it would be desirable to provide such a process which produces a stable, functional, protein product which is particularly useful for human consumption. Such a process would permit its operation at will rather than require initiation of the process very shortly after the animal source is killed so that processing can be extended over a desired time schedule.
This invention is based upon our newly discovered properties of the myofibrillar and sarcoplasmic proteins of muscle tissue which permit their processing at low pH, below about 3.5. Muscle tissue (fish or meat) is disrupted to form particles, such as by being ground or homogenized with enough water and at a pH to solubilize a major proportion, preferably substantially all of the available protein. Solubilization is effected at a low pH below about 3.5, but not so low as to effect substantial destruction of proteins, preferably between about 2.5 and about 3.5. During the solubilization step, the myofibril and sarcomere tissue structure is substantially completely converted to solubilized protein so that the final product obtained as described below is substantially free of the myofibril and sarcomere tissue structure. This process differs from the conventional process for making surimi in that major myofibrillar proteins are never solubilized in the conventional process. In the conventional process, of making surimi myofibrillar proteins are simply washed in water or in water that has been made slightly alkaline to remove water-soluble materials that lead to loss of quality of the product. Unfortunately, this conventional process also removes water-soluble sarcoplasmic proteins.
In an optional embodiment of this invention, the disrupted muscle issue can be mixed with an aqueous solution to give a pH typically between about 5.0 and about 5.5 to provide a suspension of muscle particles which can be more easily treated to solubilize proteins in the subsequent low pH treatment step to produce a solution having a sufficient low viscosity, i.e, a non-gel, so that it can be easily processed. By conducting this optional preliminary step at pH between about 5.0 and about 5.5, a homogeneous suspension is obtained wherein the protein does not imbibe excessive concentration of water. Thus, reduced volumes of water are processed which must be treated to effect the desired lower pH in the subsequent solubilization step.
The solubilized protein material from the low pH treatment step, then is treated to precipitate the proteins such as by raising its pH to between about 5.0 and about 5.5, addition of salt, the combination of salt addition and increase in pH, the use of a coprecipitant such as a polysaccharide polymer or the like to recover an insoluble protein product containing myofibrillar proteins and a significant proportion of the sarcoplasmic protein of the original muscle tissue proteins in the original muscle tissue process feed. xe2x80x9cThe protein product can contain membrane protein present in the original animal tissue process feed.xe2x80x9d Also, as set forth above, the precipitated protein is substantially free of myofibril and sarcomere tissue structure. Myofibrils and sarcomere tissue comprise strands of tissue or portions of tissue strand structure which can be viewed under a microscope. Myofibrils and sarcomere are formed primarily of proteins.
In an alternative process of this invention, the muscle tissue can be washed to obtain an aqueous solution of sarcoplasmic protein. This solution is treated at low pH as set forth above and then precipitated as set forth above in the presence of myofibrillar protein.
In an alternative process, this precipitation step need not be conducted to recover the protein product. The protein product can be treated directly without raising its pH such as by precipitation with a salt, polymer or the like and can be spray dried to be used, for example, in acidic foods. Alternatively, the low pH protein-rich solution can be treated to improve its functional properties, such as with an acidic proteolytic enzyme composition or by fractionating the protein.
The precipitated protein composition recovered at the higher pH condition can be further treated to produce a food product. Such further treatment can include lyophilization, freezing with or without an added cryoprotectant composition and with or without raising its pH or gelation by raising its pH.